1. Field of the Invention
The present invention relates to a brake control system for a vehicle, which performs various controls including a steering control by braking to restrain an excessive oversteer and excessive understeer which will occur during, for example, cornering, by applying a braking force to each wheel of the vehicle irrespective of depression of a brake pedal. More particularly, the present invention relates to the brake control system having a hydraulic pressure control apparatus for discharging brake fluid of a master cylinder by a hydraulic pressure pump through a modulator, and storing the brake fluid drained from a wheel brake cylinder through the modulator, in a reservoir.
2. Description of the Related Arts
Recently, in order to control a vehicle motion characteristic, especially to control a turning characteristic, it has been proposed to control directly a turning moment by controlling braking force applied to left and right wheels differently. In the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling vehicle motion through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle has been proposed. The apparatus is constituted so as to control the braking force applied to the vehicle by the braking force control system in response to a comparison of a desired yaw rate with an actual yaw rate, thereby to improve the vehicle stability during the course of vehicle motion such as cornering. Consequently, a braking force is applied to each wheel irrespective of depression of a brake pedal, so that a so-called steering control by braking is performed to provide an oversteer restraining control and understeer restraining control.
Also, there is known a traction control system (TRC), by which the braking force is applied to a driven wheel, when an acceleration slip occurs while the brake pedal is not depressed. For example, a Japanese Patent Laid-open Publication No. 64-74153 discloses a hydraulic pressure control apparatus for use in the traction control system. In FIG. 3 of this publication, there is disclosed an example of the apparatus, in which a single TRC changeover valve is connected to an ABS circuit of a hold circuit. A Japanese Patent Laid-open Publication No. 5-116609 discloses a hydraulic pressure control apparatus for use in a vehicle with all wheels driven, which includes two brake circuits, an ABS for providing an individual control for wheels by means of ABS valves, a pump of a self-supply type disposed in each brake circuit for returning the brake fluid drained when the control is performed, and a brake ASR using a return pump for applying the braking pressure in case of the ASR. In this publication, it was aimed to perform the braking operation, even when the ASR is being performed. With respect to an embodiment disclosed in FIG. 1 of the publication, it is described that a changeover valve 3 and a load valve 14 are provided for the ASR, and that the load valve 14 is disposed in a connecting passage between a primary brake cylinder (in 6) and an inlet of a pump, and that a check valve separates this passage from a storage chamber 9. It is also described that when one of two wheels tends to slip, valves 3 and 14 are changed over and a pump 10 starts to be driven, simultaneously.
The hydraulic pressure control apparatus disclosed in FIG. 3 of the Japanese Patent Laid-open Publication No. 64-74153 relates to the one which controls the ABS circuit when the brake pedal is not depressed. If that apparatus was so constituted to enable the ABS circuit to be controlled when the brake pedal is depressed, the steering control by braking could be performed, thereby to enable the vehicle motion control to be performed. However, in the case where the steering control by braking for the vehicle is performed by the hydraulic pressure control apparatus as disclosed in FIG. 3 of that publication, for example, the brake fluid stored in the reservoir can not be drained, due to pressure increase which will occur when the brake pedal is depressed, thereby to fulfill the reservoir. As a result, it may be difficult to perform a desired pressure decreasing operation when the ABS (anti-skid control) starts.
According to the above-described publication No. 5-116609, as for the changeover valve 3 and load valve 14, a pair of two-port two-position solenoid operated switching valves are provided, and adapted to be changed over simultaneously. Therefore, in view of the embodiment as disclosed in FIG. 1 of that publication, there may be a case where the brake fluid in the reservoir can not be drained, thereby to fulfill the reservoir. In this case, however, no means for draining the fluid appropriately has been provided.
In addition, recently proposed is a brake-assist control, which compensate for lack of depressing force applied by a vehicle driver in case of an emergency braking operation. The brake-assist control is made by pressurizing the brake fluid automatically in response to braking conditions such as a brake pedal speed, thereby to apply the desired braking force. According to this apparatus, if the apparatus includes the hydraulic pressure pump and reservoir, and is adapted to suck the brake fluid in the reservoir or the master cylinder, then the same problem will occur as described above. Although it is possible to provide a sensor for detecting the brake fluid stored in the reservoir, and determine whether the brake fluid in excess of a predetermined volume remains in the reservoir, through the sensor. However, this sensor is very expensive, so that it is preferable to avoid using this sensor, while wheel speed sensors are needed for all of the controls to be performed.